Generally, as a fixing apparatus used in an electrophotographic image forming apparatus such as a copying machine, a printer, and the like, a heat roller type fixing apparatus is frequently used. The heat roller type fixing apparatus includes a fixing roller and a pressing roller which are pressed against each other, wherein heating means constituted of a halogen lamp (halogen heater) disposed in the fixing roller or halogen lamps (halogen heaters) disposed in both the rollers heats the pair of rollers at a predetermined temperature (fixing temperature), and a recording paper on which an unfixed toner image is formed is fed to a pressing section (fixing nip section) of the pair of rollers, and the recording paper is allowed to pass through the pressing section so as to fix the toner image by heat and pressure.
In a fixing apparatus provided in a color image forming apparatus, it is general to use an elastic roller having an elastic layer which is made of silicon rubber or the like and which is provided on a fixing roller surface layer.
The elastic roller is used as the fixing roller, so that the fixing roller surface is elastically deformed corresponding to an uneven surface of the unfixed toner image and is in contact with the toner image so as to cover the toner image. This allows the color unfixed toner image whose toner amount is larger than that of monochrome to be favorably heated and fixed. Further, due to strain release of the elastic layer which occurs in the fixing nip section, it is possible to improve a releasing property with respect to color toner which is more likely to be offset than monochrome toner. Further, a nip shape of the fixing nip section has a concave upward (on the side of the fixing roller) (that is, the nip shape is a so-called inverse nip shape), so that it is possible to more favorably strip paper. As a result, it is possible to strip paper without using any stripping means such as a stripping protrusion (self stripping), so that it is possible to prevent insufficient image formation which is caused by the stripping means.
Note that, an image forming apparatus using monochrome toner is also arranged so that the elastic layer is provided only on the pressing roller without being provided on the fixing roller so as to secure the fixing nip.
Incidentally, the fixing roller or the pressing roller which includes the elastic layer raises the following problem. In the fixing roller having the elastic layer, the elastic layer cannot sufficiently conducts heat. Thus, in case where the heating means is provided in the fixing roller, heat is less efficiently conducted, so that it takes longer time to warm up. In case of allowing sheets to sequentially pass at high speed, the temperature of the fixing roller drops.
As a method for solving these problems, a technique in which external heating means is brought into contact with the fixing roller surface so that the fixing roller is heated from the outside (external heat fixing process) is known.
For example, Patent Document 1 (Japanese Unexamined Patent Publication No. 198659/2004 (Tokukai 2004-198659) (Publication date: Jul. 15, 2004)) proposes a technique (an external belt heat fixing process) using an external heating belt (endless belt), suspended by belt suspending rollers, as external heating means. In this technique, the external heating belt is used as the external heating means, so that a contact area between the external heating means and the fixing roller increases, thereby promoting supply of heat from the external heating means to the fixing roller.
Further, Patent Document 2 (Japanese Unexamined Patent Publication No. 154529/2001 (Tokukai 2001-154529) (Publication date: Jun. 8, 2001)) discloses a technique in which: a roller (external heating roller) is used as the external heating member so as to change surface temperature of the external heating member and/or the fixing roller, a type of a transfer medium, and a timing at which the external heating member is brought into contact with the fixing roller surface in accordance with an image forming mode, thereby controlling the surface temperature of the fixing roller into a most suitable value.
However, in the technique of Patent Document 1, the external heating belt is heated by heating the belt suspending rollers and heat is supplied to the fixing member via the external heating belt, so that it is necessary to set temperature of each of the belt suspending rollers and the external heating belt to be higher than the surface temperature of the fixing roller. Thus, also after stopping the heating of the belt suspending rollers, each belt suspending roller has higher temperature than the surface temperature of the fixing roller for a while. As a result, when the rotation of the fixing roller is stopped at the time of fixing-standby after finishing the fixing, a contact portion between the fixing roller and the external heating belt (particularly, a portion being in contact with the belt suspending roller via the external heating belt) is locally heated, so that the surface of the fixing roller has uneven temperature, which results in uneven gloss of an image. Note that, Patent Document 1 describes an arrangement in which the external heating belt can be brought into contact with and can be separated from the fixing roller, but this arrangement is made in order to separate the external heating belt so that a jam process or belt cleaning can be carried out. Thus, separation carried out in order to control the temperature of the fixing roller is not described in Patent Document 1 at all.
Further, the fixing apparatus of Patent Document 2 is arranged so that the external heating roller can be separated from and can be brought into contact with the fixing roller. Thus, it is possible to separate the external heating roller from the fixing roller at the time of fixing-standby. However, in the technique of Patent Document 2, the roller is used as the external heating means, so that a contact area between the external heating means and the fixing roller is small, which results in such a problem that it is impossible to sufficiently supply heat to the fixing roller.
Thus, the fixing apparatus adopting the heat fixing method may be arranged so that the external heating belt can be separated from the fixing roller so as to control the surface temperature of the fixing roller. However, in the fixing apparatus adopting the heat fixing method, it is necessary to provide a regulating member for regulating side positions of the external heating belt (positions of both sides of the external heating belt which are in a rotational axis direction of the belt suspending roller) so as to stably rotate the external heating belt within a predetermined range with respect to an axis direction of the belt suspending roller.
However, it is general that the regulating member is provided so as to be integrally rotatable and movable with the belt suspending roller in order to prevent the external heating belt from receiving excessive stress caused by sliding against the regulating member. Further, each of the external heating belt and the belt suspending roller thermally extends and shrinks depending on temperature change at the time of fixing operation and at the time of fixing operation stoppage, so that the length of the belt suspending roller changes in its axis direction. Thus, the arrangement in which the external heating belt can be separated from the fixing roller in order to control the surface temperature of the fixing roller raises such a problem that: the regulating member moves to a position being contact with a peripheral surface of the fixing roller, so that it is impossible to suitably control relative positions of the fixing roller and the belt thereafter.
This problem is further detailed as follows. FIG. 10(a) illustrates an arrangement of an external heating device 100 in which a regulating member for regulating a side position of the belt is provided on the belt suspending roller so that the side position of the external heating belt is regulated. The external heating device 100 is arranged so that: an external heating belt 103 is suspended by two belt suspending rollers 101 and 102, and the external heating belt 103 is brought into contact with the fixing roller 109. On each side of the belt suspending roller 101, a belt regulating member 104 is provided. On each side of the belt suspending roller 102, a belt regulating member 105 is provided. Thus, when the external heating belt 103 moves in a rotational axis direction of each of the belt suspending rollers 101 and 102, sides of the external heating belt 103 are respectively brought into contact with the belt regulating members 104 and 105, so that movement of the external heating belt 103 is regulated.
In the external heating device 100, in case where the external heating belt 103 is separated from the fixing roller 109 as illustrated in FIG. 10(b), when a distance between each of the belt suspending rollers 101 and 102 and (ii) the fixing roller 109 is large, rotation of the external heating belt 103 causes the belt suspending rollers 101 and 102 to move in the rotational axis directions of both the rollers as illustrated in FIG. 10(c), which may cause the belt regulating members 104 and 105 to move to positions being contact with the peripheral surface of the fixing roller 109. In this case, even in case of trying to bring the belt suspending rollers 101 and 102 into contact with the fixing roller 109, the belt regulating member 104 intervenes between the belt suspending roller 101 and the fixing roller 109, and the belt regulating member 105 intervenes between the belt suspending roller 102 and the fixing roller 109, so that it is impossible to bring the external heating belt 103 into contact with the fixing roller 109. As a result, it is impossible to supply heat from the external heating belt 103 to the fixing roller 109.
Note that, in order to prevent such a problem, it may be so arranged that a distance between the belt regulating member 104 and the fixing member 109 is increased and a distance between the belt regulating member 105 and the fixing member 109 is increased as illustrated in FIG. 10(d). However, in this case, it is necessary to increase the width of the external heating belt 103 in the axis direction in order to stably rotate the external heating belt 103 within a predetermined range with respect to the axis directions of the belt suspending rollers 101 and 102. Thus, the external heating belt 103 has a wider area, which is not in contact with the fixing roller 109, in its each side. Further, the area does not allow heat to be conducted from the external heating belt 103 to the fixing roller 109, so that the area has extremely high temperature. As a result, thermal deterioration of the external heating belt 103 is accelerated, so that its durability drops. If the thermally deteriorated area comes into contact with each of the belt regulating members 104 and 105, the area is broken.
Further, when the external heating belt 103 is separated from the fixing roller 109 in an arrangement in which a center distance between the belt suspending roller 101 and 102 is fixed, a less tension is exerted to the external heating belt 103 as illustrated in FIG. 10(b), so that the suspended belt greatly deviates from a common tangent L of the belt suspending rollers 101 and 102 (the external heating belt 103 becomes loose). While, the external heating belt 102 is rotated by the fixing roller 109 while shifting from a contact state to a separation state, so that the external heating belt 103 is rotated in a loose state. As a result, the external heating belt 103 is likely to run upon the regulating members 104 and 105. If the external heating belt 103 run upon the regulating members 104 and 105, left tension and right tension of the external heating belt 103 (tension distribution of the belt suspending rollers 101 and 102 in the rotational axis direction) are uneven, so that a great deviation force in the axis direction is exerted to the external heating belt 103. As a result, the external heating belt 103 is broken.
Note that, in order to prevent the external heating belt 103 from being loose even in case where the external heating belt 103 is separated from the fixing roller 109, it may be so arranged that: the belt suspending roller 102 is used as a tension roller, and the center distance between the belt suspending rollers 101 and 102 is made variable so as to continuously exert a tension to the external heating belt 103. However, in this case, it is necessary to move the belt suspending roller 102 (tension roller) in separating the external heating belt 103, and it is necessary also to provide a mechanism for pressing the belt suspending roller 102. This results in a larger size of the fixing apparatus and in a more complicate structure of the apparatus.
Further, in order to solve the problems, it may be so arranged that the belt regulating member (snaking prevention rib) is provided on an internal peripheral surface of the external heating belt 103. However, in this case, it is necessary that the temperature of the external heating belt 103 is equal to or higher than the surface temperature of the fixing roller 109 in order to supply heat to the fixing roller 109, so that the snaking prevention rib is exposed to the high temperature. As a result, it is difficult to keep the adhesive strength for an extended period of time.